Various arrangements of supply units for lighting devices, for example solid-state lighting devices such as devices using LED sources as light radiation sources, may resort to switching topologies. This may be implemented, for example, in insulated power supply arrangements known as Power Supply Units (PSUs).
In high-end applications, both the primary and the secondary side of the PSU may include a microcontroller, with the need to exchange information across the insulation bather between primary side and secondary side.
Passing information may involve critical aspects, for example if performance-limited optocouplers are used.
In various implementations, the information exchange between primary and secondary side may include, in addition to a low-speed bidirectional data exchange (adapted to pass numerical information concerning slow-changing parameters), a wide-bandwidth channel adapted to transmit a high-speed signal which is used for control functions, which is able for example to provide the primary side with a feedback signal from the secondary side output, in order to close the regulation loop.
Considering by way of example the transmission going from the secondary to the primary side, the need may arise to transmit two types of information across the insulation bather:
quasi-static parameters (for instance various setting points, temperatures, system states and so on) which must be transmitted through slowly-changing signals, which do not require a high transmission speed, and
a wide-bandwidth signal, which is needed to close the regulation loop and achieve proper electrical values at the PSU output.
Various implementations based on the use of optocouplers may comply with two basic arrangements.
A first arrangement involves the separation of an analog feedback from a digital data communication. Such an architecture may require two different optocouplers operating from the secondary to the primary side: one operating in a digital way, the other transmitting a linear current, so as to fully employ the small-signal bandwidth of the optocoupler.
This solution may involve both advantages and disadvantages:
the bilateral digital data exchange may be based on standard low-speed optocouplers, adapted to be driven and decoded by standard UART (Universal Asynchronous Receiver Transmitter) peripherals, which are available also on low-cost microcontrollers;
as previously mentioned, in order to transmit from the secondary to the primary side it may be necessary to use two optocouplers, and this may double costs and the space needed on the PCB (Printed Circuit Board) compared to solutions employing only one optocoupler;
the optocoupler acting on the analog signal may be affected by a remarkable variability of its Current Transmission Ratio (CRT).
Some implementations may involve the creation of a high-speed serial channel from the secondary to the primary side, with the following consequences:
only one optocoupler may be used (from the secondary side to the primary side);
such a fast coupler may be costly, power consuming and physically bigger compared to standard optocouplers; it is a component which is not easily available as a fast device adapted to operate at high insulation voltages;
due to the use of a standard asynchronous protocol, both communicating microcontrollers must be fitted with high data-rate UARTs, which are rarely available inside low-cost ICs; moreover, the serial codec process causes a notable CPU overhead;
such a high data-rate communication normally suffers from the noise generated by the switching electronics of the power section. High-speed signals are often affected by power switching proximity and self-disturbance events.